1. Field of the Invention
The present invention relates to a High Speed Downlink Packet Access (HSDPA) communications system and, more particularly, to an apparatus and method for selecting a Redundancy Version (RV) of a Hybrid Automatic Retransmission reQuest (HARQ).
2. Description of the Related Art
As an evolution of the asynchronous IMT-2000 standard, the HSDPA refers to a packet dedicated access technique used for high speed downlink data transmissions and is a main feature of the Release 5 standards specification of the 3GPP (3rd Generation Partnership Project). In particular, HSDPA is recognized as an effective technique for providing Internet Protocol (IP) multimedia services, which has been recently gaining much attention with the remarkable enhancement of data transmission rates.
A core technology of HSDPA for improving transmission efficiency includes Adaptive Modulation and Coding (AMC), HARQ, and Node B scheduling. In order to effectively use the link adaptive techniques adopted for HSDPA, such as AMC or HARQ, control elements or components to control these functions need to be physically located as close to the wireless access interface as possible.
In addition, if an element for handling the data scheduling is located in a radio network controller (RNC), problems related to delays would prevent proper scheduling from being performed for the appropriate channel environment. Thus, for HSDPA, most radio resource control functions including the scheduling function are preferably handled by the base station (Node B), rather than by the RNC.
The HARQ technique aims to enhance the transmission efficiency of packet data by combining FEC (Forward Error Correction) together with ARQ techniques. Specifically, HARQ is a method that combines the characteristics of FEC, such as correcting the erroneous data up to a certain range when transmitted data contains an error, and the characteristics of the ARQ, such as re-transmitting data transmission failures.
Accordingly, in a system employing HARQ, when a reception side, or receiver, fails to recover a data packet, it requests re-transmission of the corresponding packet from a transmission side, or transmitter. In order to enhance error recovery performance of the system, the reception side combines newly received packet data and stored packet data before performing a decoding process. As used herein, “re-transmission” refers to packet re-transmission for recovering error-generated data.
In a WCDMA (Wideband Code Division Multiple Access) HSDPA system, an output of the turbo encoder includes systematic bits carrying row data, such as user information, and parity bits, or redundancy information, carrying information created based on the user information. In addition, in the WCDMA HSDPA system, eight Redundancy Versions (RVs) can be selected in performing re-transmissions according to the HARQ method and the eight RVs have a different data structure, respectively.
The data structure of the RV is determined by a combination of two parameters, such as “s” and “r,” or three parameters, such as “s,” “r” and “b.” Among the parameters, the “s” parameter is used for allocating priority to either the systematic bits or the parity bits of a turbo code, the “r” parameter is used for puncturing and repetition and the “b” parameter is used for changing a position of constellation. The “b” parameter is applicable only for 16QAM (Quadrature Amplitude Modulation).
In the WCDMA HSDPA system, the throughput of the system differs depending upon which combination of RVs is selected in the re-transmission. Therefore, the RVs are selected in order to maximize the system throughput in each re-transmission.
In re-transmission, an MCS (Modulation and Coding Selection) level is not changed. Therefore, in order to obtain a maximum throughput, an RV that can minimize a BLER (Block Error Rate) must be selected. In this respect, the RV that can minimize the BLER differs depending upon each MCS or upon the number of times of re-transmission.
Accordingly, in order to measure the throughput of the system and select an RV that can maximize the system throughput in the related art HARQ method, simulation must be performed on every MCS received from the reception side, for example, a terminal, according to the number of times re-transmission is performed. However, since the related art HARQ method performs the simulation with respect to every MCS according to the number of times re-transmission is performed, a large number of simulations must be performed. Furthermore, if the specification or particulars of the system is changed, the large number of simulations must be performed again.
Therefore, there is a need for an apparatus and method for selecting an RV to maximize throughput when performing HARQ transmissions without requiring a large number of simulations to be performed. The present invention addresses these and other needs.